Method for making spectrally efficient photodiode structures for CMOS color imagers

ABSTRACT

A method for making an array of photodiodes with more uniform optical spectral response for the red, green, and blue pixel cells on a CMOS color imager is achieved. After forming a field oxide on a substrate to electrically isolate device areas for CMOS circuits, an array of deep N doped wells is formed for photodiodes for the long wavelength red pixel cells. An array of P doped well regions is formed adjacent to and interlaced with the N doped wells. Shallow diffused N +  regions are formed within the P doped wells for the shorter wavelength green and blue color pixels cells. The shallow diffused photodiodes improve the quantum efficiency (QE), and provide a color imager with improved color fidelity. An insulating layer and appropriate dye materials are deposited and patterned over the photodiodes to provide the array of color pixel cells. The N and P doped wells are also used for the supporting FET CMOS circuits to provide a cost-effective manufacturing process.

BACKGROUND OF THE INVENTION

[0001] (1) Field of the Invention

[0002] The present invention relates to the fabrication of a CMOS colorimager on a semiconductor substrate using CMOS technology, and moreparticularly relates to a method for making photodiodes with improved(more uniform) spectral response across the frequency band for the red,green, and blue (wavelength) pixel cells in the array of photodiodes.

[0003] (2) Description of the Prior Art

[0004] In camcorders and early digital cameras, charge coupled devices(CCDs) were used as optical detectors for detecting and processing colorimages. The CCDs detect the light on an array of photosites on thesurface of an image sensor, and then the induced charge at each site ina row of the array of photosites is sequentially transferred to areadout register where it is detected, amplified, and processed throughan analog/digital converter and stored as digital information forfurther processing. Although the CCD is a convenient device forphotoimaging, it is not as economically practical as the CMOS imagesensor for integrating other (camera) circuit functions on the samesensor chip for image processing. Since the CMOS technology is moreadvanced because of its extensive use in computers for logic functions,such as central processing units (CPUs) and for data storage and thelike, the CMOS technology is the technology of choice for color sensorsin future digital cameras. The CMOS technology allows more function tobe integrated directly on the sensor chip. Therefore fewer CMOS chipsare required than for the CCD technology, and the CMOS color imager ismore manufacturing cost effective.

[0005] To better understand the limitations of the present CMOS colorimager technology, a portion of a CMOS color imager, having photodiodesfor the primary color pixels for red, green, and blue (R/G/B)wavelengths, is depicted in the schematic cross-sectional view ofFIG. 1. In the conventional process, a single diffused photodiode istypically used for each of the colored (R/G/B) pixels. These photodiodesare formed in the substrate 10 by forming N doped wells 12, one for eachcolor pixel, in an array of pixels. FIG. 2A shows one of the CMOScircuits associated with each photodiode, commonly referred to as anactive photodiode circuit. The function of the circuit is to sample thechange in output voltage (delta V_(out)) on the photodiode at the outputnode 22 that is a function of the light intensity (number of photonsnhv) impinging on the surface of the diode 12. Briefly, the photodiodes12, one of which is shown in FIG. 2A, are reversed biased and the diodesare charged to a reset voltage V_(reset). When an optical image isimpressed on the photodiodes, the light intensity is measured as thenumber n of photons (having energy=hv) 20 generating the photocurrentI_(ph) and charging the diode 12. The h is Planck's constant. The v isthe frequency of the light, and is related to wavelength lamda bylamda=c/v, where c is the speed of light. The diode 12 is theninterrogated using the row select (row_s) gate 24 to determine thechange in voltage delta V at the output voltage V_(out), which isproportional to the light intensity or energy nhv. In FIG. 2B this deltaV is depicted as the change in V_(out) in the chart for voltage vs.time, where the vertical axis is a measure of the change in voltage(delta voltage) and the horizontal axis is time in milliseconds. Thereset voltage V_(reset) 26 waveform is shown offset above the plot ofthe change in output voltage (delta V_(out)) 22′ also plotted in FIG.2B. After interrogating the array of photodiodes (pixels) and processingand storing the digital data, the active photodiode circuit is thenreset for recording the next optical image. The primary colors (R,G,B)in the image imposed on the CMOS image sensor are determined by using aseparate diode for each of the color pixels. This is achieved by usingcolor filters (or dyes), that is red, green, and blue (R/G/B) filter16′, 16″, and 16′″, respectively, over each of the three diodes, asdepicted in FIG. 1.

[0006] The color filter response curve for the photodiodes 12 with thethree color filters 16′ (red), 16″ (green), and 16′″ (blue) of FIG. 1used to detect the color image is depicted in FIG. 3A. The color filterresponse is also shown in arbitrary units. The ideal or preferredresponse profile is shown as the solid curves 4, and the actual responseprofile for a conventional photodiode 12 is depicted by the dashedcurves 6′, 6″, and 6′″, respectively, for the red, green, and blue pixelcells 16′, 16″, and 16′″ of FIG. 1. It is clearly seen that the colorfilter response is substantially reduced at the shorter wavelength bluepixel cells 6′″ at a wavelength of 450 nm. This results in poor colorfidelity of the original color image. This poor color fidelity is aresult of the nonuniform quantum efficiency (QE) across the opticalwaveband from 450 to 650 nm. This variation in QE is depicted by thecurve 2 in FIG. 3B, where the y-axis is the QE (the ratio of the numberof photoelectrons to the number of photons) measured in arbitrary units,and the x-axis is the wavelength of the light in nm.

[0007] Several methods of forming photodiodes for CMOS color imagershave been reported in the literature. For example, Drowley et al., U.S.Pat. No. 6,023,081, describe a semiconductor image sensor in which thered and blue pixels are made in the same type of well area (N well). Theblue and red pixels are made after forming the FET gate electrodes. InU.S. Pat. No. 5,965,875 to Merrill a triple diffused well structure isused to separate out and to detect and measure the intensity of thethree primary colors red, green, and blue. The method is based on theprinciple that the absorption length of the light in silicon is afunction of the different frequencies. U.S. Pat. No. 6,040,593 to Parket al. describes a method for making a buried diffused photodiodestructure with a self-aligned silicide layer for making CMOS imagesensors. U.S. Pat. No. 5,122,850 to Burkey describes a method for makingCCD image sensors, which include P-stripes (diffused regions) under thetransfer gate of the CCD devices and adjacent to but not touching thephotodiode which provides effective anti-blooming control whileeffectively transferring the photocharge to the CCD. U.S. Pat. No.5,514,620 to Aoki et al. describes a method of using solid statediffusion for making shallow PN junction devices that includephotoelectric conversion devices.

[0008] However, there is still a strong need in the digital imagingindustry to provide photodiodes for color imagers with a more uniformspectral response curve (QE) across the optical spectral range for red,green, and blue pixels (photodiodes).

SUMMARY OF THE INVENTION

[0009] A principal object of this invention is to provide photodiodesfor color imagers with a more uniform spectral response (QE) for thered, green, and blue (R/G/B) pixels for better color fidelity.

[0010] It is another object of this invention to improve the spectralresponse by varying the individual junction depths of the diffusedphotodiodes for the R/G/B pixels to provide a more uniform spectralresponse curve for the optical bandwidth.

[0011] Still another object of this invention is to form this moreuniform spectral response curve by forming deep photodiode diffused(metallurgical) junctions for the longer wavelength red pixels, andshallower diffused photodiode junctions for the shorter wavelength greenand blue pixels.

[0012] A further object of this invention is to provide a process thatis compatible with the standard CMOS processes to form CMOS colorimagers for a more cost-effective manufacturing process.

[0013] In accordance with the above objects, a method for fabricatingphotodiodes for CMOS color imagers with a more uniform spectral responsefor the red, green, and blue pixel cells is now described. The methodutilizes photodiodes with different diffused junction depths to modifythe quantum efficiency (QE) of the photodiodes. The method can be usedwith existing CMOS process technology, and therefore the CMOS colorimager can include additional signal processing circuits integrated intothe existing CMOS imager to reduce cost.

[0014] The method for forming these photodiodes for CMOS color imagersbegins by providing a semiconductor substrate consisting of a P⁻ dopedsingle-crystal silicon. Each CMOS color imager (chip) formed on thesubstrate consists of device areas for CMOS circuits and optical deviceareas for an array of photodiodes for the alternating red, green, andblue pixel cells. Typically the device areas are surrounded andelectrically isolated by field oxide areas. An array of N doped wells isformed, for example, by implanting phosphorus ions (P³¹) in the opticaldevice areas for the photodiodes in the red pixel cells. An array of Pdoped wells is formed adjacent to the N doped wells, for example, by ionimplanting boron ions (B¹¹). A key feature of this invention is to formshallow doped N⁺ regions in the P doped wells, for example, byimplanting arsenic ions (As⁷⁵). Since the photon absorption depth in thesilicon is a function of the photon wavelength, the shorter wavelengthgreen and blue light has a shallower absorption depth. The absorption ofthe blue photons in the shallower N⁺ doped photodiodes results inenhanced photocurrent I_(ph) and in improved quantum efficiency. Theshallow diffused N⁺ doped photodiodes formed in the P doped wells arethen used to make the green and blue pixel cells. Dye materials aredeposited for the red, green, and blue filters. The individual dyematerials are patterned to form appropriate optical filters over thered, green, and blue pixel cells thereby completing the photodiodes forthe CMOS color imager.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The objects and other advantages of the invention will becomemore apparent in the preferred embodiment when read in conjunction withthe following drawings.

[0016]FIG. 1 shows a schematic cross-sectional view of a portion of aCMOS color imager through three photodiodes used to form the red, green,and blue pixel cells for a conventional imager of the prior art.

[0017]FIG. 2A shows a schematic circuit for a conventional (prior art)active photodiode circuit associated with each photodiode.

[0018]FIG. 2B is a graph of the photodiode voltage as a function of timeshowing the reset voltage V_(reset) and the change in the output voltage(delta V) as a result of the photocurrent I_(ph) induced by impingingphoton energy (nhv) during the imaging for the circuit of FIG. 2A.

[0019]FIG. 3A shows a plot of the color filter response for theconventional (prior art) photodiodes as a function of the blue, green,and red wavelengths of 450, 550, and 650 nm, respectively.

[0020]FIG. 3B shows a graph of the quantum efficiency (QE) as a functionof the optical wavelength for the conventional photodiodes of the priorart.

[0021]FIG. 4 shows a schematic cross-sectional view of a portion of aCMOS color imager through three photodiodes used to form the red, green,and blue pixel cells, by the method of this invention, with photodiodeshaving varying diffused junction depths.

[0022]FIG. 5 shows a graph of the simulated quantum efficiency as afunction of optical wavelength for the photodiodes with the deepdiffused junctions.

[0023]FIG. 6 shows a graph of the simulated quantum efficiency as afunction of optical wavelength for the photodiodes with the shallowdiffused N⁺ junctions in the P doped wells.

[0024]FIG. 7A shows a plot of the color filter response for the improvedphotodiodes of this invention as a function of the blue, green, and redwavelengths of 450, 550, and 650 nm, respectively.

[0025]FIG. 7B shows a graph of the improved QE as a function of theoptical wavelength for both the shallow and deep diffused photodiodes ofthis invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0026] In accordance with the objects of this invention, a method formaking spectrally efficient photodiodes with more uniform spectralresponse for color imagers is now described in detail. Although thefabrication of the P- and N-channel FETs are not described in detail, itshould be understood that these novel photodiodes are integrated intothe sensor circuit using some of the same FET process steps to reduceprocess cost. For example, the N doped and P doped wells in the CMOScircuits are also used in fabricating the photodiodes with a moreuniform spectral response.

[0027] Referring to FIG. 4, a schematic cross section through a portionof a color imager is shown. The cross section is through photodiodes foronly three color pixels of the array of pixels in the color imager. Thephotodiodes are formed in a semiconductor substrate 10. The substrate 10is typically a P doped single-crystal silicon having a <100>crystallographic orientation. Prior to forming the photodiodes, a fieldoxide is formed on and in the substrate to electrically isolate deviceareas for the CMOS circuits. However, the field oxide is not relevant tounderstanding the invention, and therefore is not explicitly shown inthe figures to simplify the drawings and the discussion.

[0028] Referring still to FIG. 4, an array of N doped wells 12(N) forthe photodiodes of the red pixels cells is formed in the P⁻ substrate10. Only one of the N doped wells 12(N) is depicted in FIG. 4. The Ndoped wells are formed by using a patterned photoresist implant mask andimplanting phosphorus ions (P³¹). The P³¹ ions are implanted and thesubstrate is annealed during subsequent processing to provide a finaldopant concentration of between about 1.0 E 16 and 1.0 E 18 atoms/cm³.The N doped wells are implanted to have a preferred depth for thediffused junction (metallurgical junctions where the N dopantconcentration=P⁻ dopant concentration) of between about 0.5 and 1.0micrometers.

[0029] Still referring to FIG. 4, an array of P doped wells 13(P) isformed adjacent to the N doped wells 12(N) in which the photodiodes forthe green and blue pixel cells are formed. The P doped wells 13 areformed by using a photoresist implant mask and implanting boron (B¹¹) tohave a final dopant concentration of between about 1.0 E 16 and 1.0 E 18atoms/cm³. The preferred depth of the diffused P doped wells is betweenabout 0.5 and 1.0 micrometers. These N and P doped wells can also serveas the wells for the P and N channel FETs of the CMOS circuits.

[0030] Still referring to FIG. 4, and a key feature of this invention,shallow diffused N⁺ doped regions 15(N⁺) are formed in the P doped wells13(P). The shallow doped regions 15(N⁺) are formed by using aphotoresist implant mask and implanting arsenic ions (As⁷⁵). Thepreferred final dopant concentration of the arsenic is between about 1.0E 20 and 1.0 E 21 atoms/cm³, and the metallurgical junction is at adepth of between about 0.05 and 0.1 micrometers in the P doped wells13(P).

[0031] Continuing with FIG. 4, an insulating layer 14 is deposited onthe array of photodiodes 12(N) and 15(N⁺). The insulating layer ispreferably an undoped silicon oxide (SiO₂) and is deposited by chemicalvapor deposition. The SiO₂ is deposited to a thickness of between about3000 and 5000 Angstroms.

[0032] Also as shown in FIG. 4, the photodiode portion of the CMOS colorimager is completed by depositing and patterning a dye material for eachof the R/G/B color pixels to form the red, green, and blue color opticalfilters 16′, 16″, and 16′″. For example, each of the dye materials canbe deposited by spin coating to form a film, as commonly practiced inthe industry, and the dye materials are patterned using conventionalphotolithographic techniques and etching.

EXAMPLE

[0033] To better appreciate the invention for making these photodiodeswith improved uniformity across the spectral range, the quantumefficiency (QE) of these photodiodes was determined as a function ofoptical wavelength. The QE was determined by simulating the collectorefficiency as a function of optical wavelength for the N wellphotodiodes 12(N) and the N⁺ photodiodes 15(N⁺) formed in the P wells13(P) for the photodiodes depicted in FIG. 4. The QE was simulated usinga Medici simulation program provided by Avanti Corp. of U.S.A. FIG. 5shows a plot of a curve 8 for the collector efficiency vs. the opticalwavelength in micrometers for the deep N well red photodiode 12(N). Asdepicted in FIG. 5, the QE is about maximum (60%) at the long wavelength(650 um) for the red light, and the QE drops off dramatically to about50% at the shorter wavelengths (450 um) for the blue light. FIG. 6 showsa plot of a curve 9 for the collector efficiency vs. the opticalwavelength in micrometers for the shallow N⁺ doped green and bluephotodiode 15(N⁺) in the P well. As depicted in FIG. 6, the QE is aboutmaximum (60%) at the short wavelength (450 um) for the blue light, andthe QE remains essentially constant for the longer wavelength (550 um)for the green light. FIG. 7B depicts a plot of curves 8 and 9 (fromFIGS. 5 and 6, respectively) superimposed in the same graph toillustrate the improved spectral uniformity using a CMOS color imagerhaving the two photodiodes, namely the deep N well diode 12(N) and theshallow N⁺ diode 15(N⁺) shown in FIG. 4. The QE is plotted as a functionof optical wavelength in nanometers (nm).

[0034] This improved spectral response is also depicted in FIG. 7A forthe red, green, and blue wavelengths (450, 550, 650 nm, respectively)using color filters over the appropriate photodiodes. The color filterresponse for the three wavelengths is depicted by the dashed curves 7′,7″, and 7′″ for the red, green, and blue pixel cells, respectively, vs.the ideal curves (solid curves) 4. The color filter response curve forthe short wavelength blue light (450 nm) using the shallow N⁺ diffusedphotodiode is substantially increased compared to the response curve 6in FIG. 3A of the prior art. This provides a CMOS color imager with amuch improved (uniform) color filter response curve for the red, green,and blue color pixel cells.

[0035] While the invention has been particularly shown and describedwith reference to the preferred embodiment thereof, it will beunderstood by those skilled in the art that various changes in form anddetails may be made without departing from the spirit and scope of theinvention.

What is claimed is:
 1. A method for making spectrally efficientphotodiodes for CMOS color imagers comprising the steps of: providing aP⁻ doped semiconductor substrate having device areas for CMOS circuits,and having optical device areas for an array of photodiodes foralternating red, green, and blue pixel cells; forming an array of Ndoped regions having various junction depths and dopant concentrationsfor said red, green, and blue pixel cells to provide an essentiallyconstant quantum efficiency between said pixel cells; depositing dyematerials and patterning to form appropriate optical filters over saidred, green, and blue pixel cells thereby completing said photodiodes forsaid CMOS color imagers.
 2. The method of claim 1, wherein saidsemiconductor substrate is single-crystal silicon doped with boron to aconcentration of between 1.0 E 14 and 1.0 E 16 atoms/cm³.
 3. The methodof claim 1, wherein said N doped regions for said red pixel cells areformed by implanting phosphorus ions to provide a final concentration ofbetween 1.0 E 16 and 1.0 E 18 atoms/cm³ and to a junction depth ofbetween about 0.5 and 1.0 micrometers.
 4. The method of claim 1, whereinsaid N doped regions for said green pixel cells are formed by implantingphosphorus ions to provide a final concentration of between 1.0 E 20 and1.0 E 21 atoms/cm³ and to a junction depth of between about 0.05 and 0.1micrometers.
 5. The method of claim 1, wherein said N doped regions forsaid blue pixel cells are formed by implanting phosphorus ions toprovide a final concentration of between 1.0 E 20 and 1.0 E 21 atoms/cm³and to a junction depth of between about 0.05 and 0.1 micrometers. 6.The method of claim 1, wherein said dye materials are deposited by spincoating.
 7. The method of claim 1, wherein said dye materials arepatterned using photolithography and etching.
 8. A method for makingspectrally efficient photodiodes for CMOS color imagers comprising thesteps of: providing a P⁻ doped semiconductor substrate having deviceareas for CMOS circuits, and having optical device areas for an array ofphotodiodes for alternating red, green, and blue pixel cells; forming anarray of N doped wells in said optical device areas for said photodiodesin said red pixel cells; forming an array of P doped wells; forming N⁺doped regions in said P doped wells, wherein said N⁺ doped regions areshallower than said N doped wells for said red pixel cells, therebyforming said photodiodes in said green and said blue pixel cells;depositing dye materials and patterning to form appropriate opticalfilters over said red, green, and blue pixel cells thereby completingsaid photodiodes for said CMOS color imagers.
 9. The method of claim 8,wherein said semiconductor substrate is single-crystal silicon dopedwith boron to a concentration of between 1.0 E 14 and 1.0 E 16atoms/cm³.
 10. The method of claim 8, wherein said N doped wells areformed by implanting phosphorus ions to provide a final concentration ofbetween 1.0 E 16 and 1.0 E 18 atoms/cm³.
 11. The method of claim 8,wherein said N doped wells are formed to have a metallurgical junctiondepth of between about 0.5 and 1.0 micrometers.
 12. The method of claim8, wherein said P doped wells are formed by implanting boron ions toprovide a final concentration of between 1.0 E 16 and 1.0 E 18atoms/cm³.
 13. The method of claim 8, wherein said P doped wells areformed to have a metallurgical junction depth of between about 0.05 and0.1 micrometers.
 14. The method of claim 8, wherein said N⁺ dopedregions in said P doped wells are doped with arsenic ions to provide afinal dopant concentration of between 1.0 E 20 and 1.0 E 21 atoms/cm³.15. The method of claim 8, wherein said N⁺ doped regions in said P dopedwells are doped to have a metallurgical junction depth of between about0.05 and 0.1 micrometers.
 16. The method of claim 8, wherein said dyematerials are deposited by spin coating.
 17. The method of claim 8,wherein said dye materials are patterned using photolithography andetching.
 18. Spectrally efficient photodiodes for CMOS color imagerscomprised of: a P⁻ doped semiconductor substrate having device areas forCMOS circuits, and having optical device areas for an array ofphotodiodes for alternating red, green, and blue pixel cells; an arrayof N doped wells in said optical device areas for said photodiodes insaid red pixel cells; an array of P doped wells having N⁺ doped regionsfor said photodiodes in said green and blue pixel cells, wherein said N⁺doped regions are shallower than said N doped wells for said red pixelcells; patterned dye materials over said red, green, and blue pixelcells to provide appropriate color filters that complete saidphotodiodes for said CMOS color imagers.
 19. The structure of claim 18,wherein said semiconductor substrate is single-crystal silicon dopedwith boron to a concentration of between 1.0 E 14 and 1.0 E 16atoms/cm³.
 20. The structure of claim 18, wherein said N doped wells aredoped with phosphorus to provide a final concentration of between 1.0 E16 and 1.0 E 18 atoms/cm³.
 21. The structure of claim 18, wherein said Ndoped wells are doped to have a metallurgical junction depth of betweenabout 0.5 and 1.0 micrometers.
 22. The structure of claim 18, whereinsaid P doped wells are doped with boron ions to provide a finalconcentration of between 1.0 E 16 and 1.0 E 18 atoms/cm³.
 23. Thestructure of claim 18, wherein said P doped wells are doped to have ametallurgical junction depth of between about 0.5 and 1.0 micrometers.24. The structure of claim 18, wherein said N⁺ doped regions in said Pdoped wells are doped with arsenic ions to have a final dopantconcentration of between 1.0 E 20 and 1.0 E 21 atoms/cm³.
 25. Thestructure of claim 18, wherein said N⁺ doped regions in said P dopedwells are doped to have a metallurgical junction depth of between about0.05 and 0.1 micrometers.